1. Field of the Invention
The present invention relates to heaters and heating elements, and more particularly, to an apparatus for supporting and for preventing a graphite heating element from contacting a ceramic base in which the heating element is mounted.
2. Description of the Related Art
Laser methods are sometimes used to separate isotopes for the enrichment of uranium. One such laser method is the Atomic Vapor Laser Isotope Separation (AVLIS) process. In the AVLIS process, the uranium is held in a vapor filled extractor having graphite walls. The graphite walls of the extractor must be heated in order to maintain the vapor within the extractor. The heating is accomplished by placing heaters along the exterior walls of the extractor. The extractor, along with the heaters attached thereto, is placed in a vacuum chamber.
Prior art extractor heaters typically include a ceramic base having a channel formed therein with a high temperature molly spring-like heating filament running through the channel. The heating filament is held in place with a ceramic grout. An electric current is drawn through the molly heating filament which causes it to become hot. The heat from the filament is transferred to the ceramic base which heats the extractor walls. This type of heater is capable of operating at approximately 5 watts per square centimeter. However, in order to achieve the 1000.degree. C.+temperatures that are required in an extractor in the AVLIS process, the heaters must be operated continuously at their maximum output for long periods of time. Such intense use tends to cause the molly filament to burn-up relatively quickly.
Graphite-graphite composites have previously been used as heating elements for heaters. Graphite heating elements are capable of withstanding higher temperatures for longer periods of time than high temperature molly filaments. The higher heat capacity of graphite heating elements makes them ideal for replacing the molly filaments in heaters used to heat extractors in the AVLIS process. However, when a graphite heating element is used in a ceramic base, a chemical reaction occurs between the graphite and the ceramic at temperatures above 1000.degree. C. The chemical reaction compromises the electrical isolation and the mechanical stability of the graphite heating element and until now has prevented graphite heating elements from being used with ceramic bases.
Thus, because ceramic is an ideal material to use for heater bases, and graphite is an ideal material to use for heating elements, there is a need for an apparatus that will prevent chemical reactions from occurring when graphite heating elements are used with ceramic bases.